Distributed optical fiber sensor system

ABSTRACT

A distribution optical fiber sensor system measures distortion and temperature of a structure with a high spatial resolution. The system has an optical fiber on an object to be measured. A light source emits a first pulse light having a pulse width longer than a transient response of an acoustic phonon and a second pulse light after a time interval during which vibration of the acoustic phonon is maintained thereby supplying the pulse lights to the optical fiber. A detector detects scattering gain spectra of a Brillouin-scattered light created in the optical fiber by the second pulse light at intervals corresponding to twice the time obtained by equally dividing the pulse width of the second pulse light. A controlling/calculating unit calculates distortion and/or temperature based on the scattering gain spectra for sections of the optical fiber corresponding to the scattering gain spectra at the respective time intervals.

TECHNICAL FIELD

The present invention relates to a distribution optical fiber sensorsystem capable of measuring a distortion created in a structure such asa bridge, a tunnel or a building and a temperature of such a structurewith a high spatial resolution using an optical fiber in view of atransitional phenomenon.

BACKGROUND ART

There have been conventionally known distribution optical fiber sensorsfor measuring a distortion distribution by measuring a frequency shiftamount of Brillouin scattering created in an optical fiber along anoptical fiber, and those for measuring a temperature distribution bymeasuring a light intensity ratio of Stokes lines to anti-Stokes linesof Raman scattering created in an optical fiber. For example, they aredisclosed, for example, on pp. 325 to 327 of the “Collection ofNext-Generation Optical Technologies” (published by Optoronics). Thespatial resolutions of these distribution optical fiber sensors have hada limit of an order of several meters due to their measuring methods.

Accordingly, an inventor of the present invention proposed adistribution optical fiber sensor system having a high spatialresolution of an order of subcentimeters in Japanese Patent ApplicationNo. H11-150618 as one of the inventors, and this application waspublished in Japanese Unexamined Patent Publication NO. 2000-074697.

FIG. 23 is a construction diagram of the distribution optical fibersensor system disclosed in Japanese Unexamined Patent Publication NO.2000-074697.

In FIG. 23, this distribution optical fiber sensor system 1000 isprovided with an optical fiber 1002, a pump light source 1003, a probelight source 1004, a light intensity detector 1005, an optical coupler1006, an optical filter 1007, a calculating means 1008, and a controlmeans 1020.

The optical fiber 1002 as a part of a sensor for detecting a distortionand a temperature is placed on a structure 1001 as an object to bemeasured. A discontinuous pump light emitted from the pump light source1003 is incident on one end of this optical fiber 1002 via the opticalcoupler 1006, whereas a discontinuous probe light emitted from the probelight source 1004 is incident on the other end of the optical fiber1002. The discontinuous pump light causes various scatterings such asBrillouin scattering, Raman scattering, and Rayleigh scattering by, forexample, the nonlinearity of the optical fiber 1002. The caused variousscatterings amplify the discontinuous probe light if the frequenciesthereof coincide with that of the discontinuous probe light, and theamplified discontinuous probe light is introduced to the optical filter1007 by the optical coupler 1006. The optical filter 1007 mainlytransmits the Brillouin-amplified discontinuous probe light(Brillouin-scattered light) from these various scattered lights. Thetransmitted Brillouin-scattered light has its light intensity detectedby the light intensity detector 1005, and a detection result isoutputted to the calculating means 1008. The control means 1020 sets thefrequency of the discontinuous probe light, controls the emissions ofthe probe light source 1004 and the pump light source 1003 so that thediscontinuous probe light and the discontinuous pump light overlap at ameasuring position on the structure 1001, controls a sampling timing inthe light intensity detector 1005 so that the Brillouin-scattered lightcreated as a result of interaction can be detected, etc.

The calculating means 1008 calculates the distortion and the temperatureof the optical fiber 1002 based on the detection result of the lightintensity detector 1005. In this calculation, the distribution opticalfiber sensor system 1000 has achieved a high spatial resolution bydividing an overlapping section where the discontinuous probe light andthe discontinuous pump light overlap into a plurality of small sections.

Since an acoustic phonon as a cause of Brillouin scattering is amechanical propagation, it cannot momentarily start vibration and atransient phenomenon is known to exist (J. Smith, A. Brown, M.DeMerchant, X. Bao, “Pulse width dependence of the Brillouin lossspectrum”, Optical Communication Vol. 168 (1999), pp. 393-398). Thus, inorder to more precisely measure a distortion and a temperature takingadvantage of Brillouin scattering, this transient phenomenon needs to beconsidered.

In view of the above problems residing in the prior art, an object ofthe present invention is to provide a distribution optical fiber sensorsystem having a high spatial resolution and taking a transientphenomenon into account by using a first and a second pump lights havingdifferent frequencies.

SUMMARY OF THE INVENTION

In order to accomplish the above object, the present invention isdirected to a distribution optical fiber sensor system, comprising anoptical fiber for sensing to be placed on an object to be measured; alight source for emitting a first pulse light having a pulse widthlonger than a transient response of an acoustic phonon and emitting asecond pulse light in succession to the first pulse after a timeinterval during which the vibration of the acoustic phonon issubstantially maintained to supply the first and second pulse lights tothe optical fiber; a detector for detecting a scattering gain spectrumof a Brillouin-scattered light created in the optical fiber by thesecond pulse light at time intervals corresponding to twice the timeobtained by equally dividing the pulse width of the second pulse light;and a calculator for calculating a distortion and/or a temperature basedon the respective scattering gain spectra at the respective timeintervals for small sections of the optical fiber corresponding to therespective scattering gain spectra at the respective time intervals.

These and other objects, features, aspects, and advantages of thepresent invention will become more apparent from the following detaileddescription of the preferred embodiments/examples with reference to theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing a construction of a distribution opticalfiber sensor system according to a first embodiment.

FIG. 2 is a graph showing a pump light outputted from an opticalfrequency converter.

FIG. 3 is a graph showing states of acoustic phonons by a first pulselight and a second pulse light.

FIG. 4 is a graph showing a frequency relationship of the respectivelights.

FIG. 5 is a graph showing a relationship between the frequency of anoscillator and a spectrum of a Brillouin-scattered light.

FIG. 6 is a diagram showing a relationship between virtual small spacesand an optical fiber.

FIG. 7 is a graph showing scattering gain spectra.

FIG. 8 is a flowchart showing operations of the distribution opticalfiber sensor system according to the first embodiment.

FIG. 9 is a flowchart showing operations in Step S17.

FIGS. 10A and 10B are graphs showing a frequency characteristic of atwo-dimensional low-pass filter.

FIG. 11 is a graph showing a relationship between measurement points andinterpolation points obtained from the measurement points.

FIG. 12 is a graph showing a matrix a (i, j) defined by equation 3 inthe case that a pulse light is rectangular.

FIG. 13 is a graph showing a characteristic of a filter used in a CTprocessing.

FIG. 14 is a diagram showing a construction of a distribution opticalfiber sensor system according to a second embodiment.

FIG. 15 is a chart showing an exemplary waveform of an input to atime-frequency analyzer 44.

FIG. 16 is a graph showing frequency spectra at time windows.

FIG. 17 is a flowchart showing operations of the distribution opticalfiber sensor system according to the second embodiment.

FIG. 18 is a diagram showing a construction of a distribution opticalfiber sensor system according to a third embodiment.

FIG. 19 is a diagram showing a construction of a distribution opticalfiber sensor system according to a fourth embodiment.

FIG. 20 is a diagram showing a construction of a distribution opticalfiber sensor system according to a fifth embodiment.

FIG. 21 is a diagram showing a physical process of the fifth embodiment.

FIG. 22 is a diagram showing a flowchart in the case of calculating alateral pressure.

FIG. 23 is a diagram showing a construction of a distribution opticalfiber sensor system disclosed in Japanese Unexamined Patent PublicationNo. 2000-074697.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

Hereinafter, embodiments of the present invention are described withreference to the accompanying drawings. In the respective figures, thesame constructions are identified by the same reference numerals and norepetitive description is given.

Construction of First Embodiment

FIG. 1 is a diagram showing a construction of a distribution opticalfiber sensor system according to a first embodiment.

In FIG. 1, a distribution optical fiber sensor system 10 of the firstembodiment is provided with a light source 11, optical couplers 12, 19,22, a controller 13, a light frequency converter 14, an RF signal source15, a light amplifier 16, an optical isolator 17, a polarizationcontroller 18, an optical fiber 21, a light receiver 23, a mixer 24, anoscillator 25, a band-pass filter (BPF) 26, an amplifier 27, ananalog-to-digital (A/D) converter 28, a buffer 29 and acontrolling/calculating unit 30.

The light source 11 feeds a light of a spectral line narrower than theline width of a Brillouin scattered light to the optical fiber 21 underthe control of the controlling/calculating unit 30 in order to measure adistortion and/or a temperature. For example, various semiconductorlasers such as a distribution feedback semiconductor laser and adistribution Bragg reflection semiconductor laser can be used as thelight source 1. In this embodiment, a semiconductor laser for emitting aCW light having a wavelength of 1550 nm (frequency at this wavelength isf0) is used. The optical couplers 12, 19, 22 are parts for coupling andbranching off incident lights and outputting the branched-off lights. Anoptical circulator may be used as the optical coupler 19.

The RF signal source 15 generates an electrical signal having a radiofrequency under the control of the controlling/calculating unit 30. Thelight frequency converter 14 is a part for converting the frequency(wavelength) of the incident light into a specified frequency and is,for example, a SSB (single side band)-LN light modulator for convertingthe frequency of the incident light in accordance with the frequency ofthe electrical signal fed from the RF signal source 15 in thisembodiment. The controller 13 controls the light frequency converter 14and the RF signal source 15 under the control of thecontrolling/calculating unit 30 to cause the light frequency converter14 to output its incident light of a specified frequency during aspecified period at a specified timing. The light amplifier 16 is a partfor amplifying the incident light up to a specified light intensity. Forexample, an optical fiber amplifier added with a rare-earth element inconformity with the wavelength of an incident light to be amplified isused as such. In the case of amplifying a light within a wavelength bandof 1500 nm, erbium (atomic number: 68) is used as the rare-earthelement. The optical isolator 17 is a part designed to transmit a lightonly in one direction.

The polarization controller 18 is a part for outputting the incidentlight after converting a polarizing surface of the incident light into aspecific polarizing surface under the control of thecontrolling/calculating unit 30. The optical fiber 21 is divided into areference light fiber portion 21-1 and a detection light fiber portion212-2 and is, for example, a quartz optical fiber. The reference lightfiber portion 21-1 is not fixed to an object to be measured, whereas thedetection light fiber portion 21-2 is fixed inside a structure (objectto be measured) 1 such as a bridge, a tunnel or a building or fixed tothe outer surface thereof. In the case of measuring only a temperaturewithout measuring a distortion, the detection light fiber portion 21-2needs not necessarily be fixed.

The light receiver 23 is a part for detecting the light intensity of theincident light and converting the detected light intensity into anelectrical signal of an intensity corresponding to the received lightintensity. The mixer 24 multiplexes a plurality of inputted electricalsignals and outputs the resulting signal. The oscillator 25 oscillatesan electrical signal having a specified frequency f1 under the controlof the controlling/calculating unit 30. The BPF 26 is a filter fortransmitting only lights within a specified frequency band. Theamplifier 27 amplifies the inputted signal to have a specifiedamplitude. The A/D converter 28 converts an analog signal into a digitalsignal. The buffer 29 temporarily saves an output of the A/D converter28. The controlling/calculating unit 30 is a part for controlling therespective parts of the distribution optical fiber sensor system 10 andcalculating the distortion and the temperature with a high spatialresolution as described later and is, for example, a personal computerincluding a microprocessor.

Next, the operation of the distribution optical fiber sensor system 10according to the first embodiment is described.

Operation of the First Embodiment

First, how a CW laser beam emitted from the light source 11 acts in thedistribution optical fiber sensor system 10 is described.

The CW laser beam having a frequency f0 and emitted from the lightsource 11 is incident on a port “a” of the optical coupler 12 anddistributed into two laser beams. One distributed laser beam isoutputted from a port “d” and incident on a port “q” of the opticalcoupler 22. The other laser beam is outputted from a port “c” andincident on the light frequency converter 14. The incident laser beamhas the frequency thereof adjusted to a specified frequency in the lightfrequency converter 14 in accordance with the radio frequency of the RFsignal source 15 and an output period of the light frequency converter14 adjusted by the controller 13.

FIG. 2 is a graph showing a pump light outputted from the lightfrequency converter, wherein a horizontal axis represents time and avertical axis represents light intensity.

The CW laser beam incident on the light frequency converter 14 from theoptical coupler 12 is converted into a first pulse light having a pulsewidth Tw1 and a second pulse light having a pulse width Tw2 inaccordance with a control signal from the controller 13 and the radiofrequency of the RF signal source 15. Since the first pulse light is apulse for starting and stabilizing the vibration of an acoustic phonon,the pulse width Tw1 thereof needs to be a duration sufficiently long toend a transient response. A period of duration of the transient responsechanges according to the material of the optical fiber and the lightintensity of the pulse, but is normally about 20 ns. Accordingly, thepulse width Tw1 is set, for example, at 100 ns with a margin in thisembodiment. Since the second pulse light is a pulse used to measure thedistortion and the temperature, the pulse width Tw2 thereof is aduration necessary for the measurement (Tw2=dt·m) and set, for example,at 30 ns in this embodiment. An interval Twr between the first andsecond pulse lights is a duration necessary to change from the frequencyof the first pulse light to that of the second pulse light and mainlydepends on the performances of the controller 13, the light frequencyconverter 14 and the RF signal source. Since the vibration of theacoustic phonon starts being attenuated upon the end of the first pulselight, the interval Twr is preferably as short as possible and set, forexample, at 2 ns in this embodiment. Since there is substantially noattenuation of the acoustic phonon at 2 ns, the interval Twrsubstantially gives no influence on the measurements of the distortionand the temperature.

The frequency f1 of the first pulse light and the frequency f2 of thesecond pulse light are set at such values that substantially similaracoustic phonons are created in the optical fiber 21 and the first andsecond pulse lights can be efficiently filtered by the BPF 26 as aresult of being multiplexed with an electrical signal having a frequencyfe of the oscillator 25 in the mixer 24. For example, in thisembodiment, the frequency f1 of the first pulse light is set at 12 GHzand the frequency f2 of the second pulse light is set at 10.8 GHz.

The first and second pulse lights outputted from the light frequencyconverter 14 are amplified in the light amplifier 16 to such an extentas to bring about a nonlinear optical effect sufficient to measure thedistortion and the temperature in the detection light fiber portion 21-2in view of losses and connection losses in the optical isolator 17, thepolarization controller 18, the optical couplers 19, 22 and the opticalfiber 21. In this embodiment, an output of the light amplifier 16 is toa maximum of 20 dBm.

The amplified first and second pulse lights have the polarizing surfacesthereof adjusted in the polarization controller 18 via the opticalisolator 17, and are incident on one port “e” of the optical coupler 19.The first and second pulse lights outputted from a port “g” of theoptical coupler 19 are incident on the optical fiber 21.

FIG. 3 is a graph showing states of acoustic phonons by the first andsecond pulse lights, wherein a horizontal axis represents positions onthe optical fiber 21. Points A, B, C and D in FIG. 3 correspond topoints A′, B′, C′ and D′ shown in FIG. 2.

In the optical fiber 21, the acoustic phonons shown in FIG. 3 arecreated by the first and second pulse lights. Upon reaching a startingpoint A′ of the first pulse light, the vibration is started in theoptical fiber 21 (point A), but the acoustic phonon gradually rises asshown in FIG. 3 (point A to point S) without being able to momentarilyrise. This transient response is about 20 ns as mentioned above. Uponthe passage of 60 ns which is three times as much (point S), theacoustic phonon is thought to reach a sufficiently stable state.Thereafter, the stable state continues up to an arrival of an endingpoint B′ of the first pulse light (point S to point B). Although thereis no light during the interval Twr between the end of the first pulselight and the start of the second pulse light, the acoustic phonon keepsits state due to inertia (point B to point C) since this interval Twr isshort. Thereafter, a starting point C′ of the second pulse light isreached. Since the acoustic phonon already vibrates in a stable state, astable acoustic phonon is created from the beginning on by the secondpulse light without any transient response. Thus, Brillouin scatteringby the second pulse light stably occurs. It should be noted that thefirst and second pulse lights create substantially the same acousticphonons. This stable state continues (point C to point D) until anending pint D′ of the second pulse light is reached. Upon the end of thesecond pulse light, the acoustic phonon is gradually attenuated toreturn to a ground state (point D to point E). In order to keep thestable acoustic phonon at point D as well, a third pulse having afrequency different from that of the second pulse may be incident on theoptical fiber 21 after a specified time interval following the end ofthe second pulse light.

The Brillouin-scattered light created by the acoustic phonons in theoptical fiber 21 is incident on the port “g” of the optical coupler 19and outputted from a port “f”. The Brillouin-scattered light outputtedfrom the port “f” is incident on a port “r” of the optical coupler 22.The CW pump light incident on the port “q” and the Brillouin-scatteredlight incident on the port “r” are coupled in the optical coupler 22 anddistributed into two lights.

FIG. 4 is a graph showing a frequency relationship of the respectivelights, wherein a horizontal axis represents frequency and a verticalaxis represent intensity. Since FIG. 4 only shows a mutual relationshipof the frequencies of the respective lights, the intensities of therespective lights differ from actual ones.

In FIG. 4, identified by frequency f0 is the CW pump light, by frequencyf1 the first pulse light, and by frequency f2 the second pulse light. Afrequency Sb1 is a frequency of the Brillouin-scattered light created bythe first pulse light, whereas a frequency Sb2 is a frequency of theBrillouin-scattered light created by the second pulse light. Arelationship: f1−Sb1=f2−Sb2=fB, holds. A band width B is a frequencyband of the light receiver 23, and ω denotes a center frequency of thelight receiver 23 and is B/2 in this embodiment. Anti-stokes lines ofthe Brillouin-scattered light and Raman-scattered light are not shown inFIG. 4 since they are irrelevant to the following description.

Here, in the case of using an optical fiber of 10.5 GHz, the followingrelationships hold: f1−f0=12 GHz, f2−f0=10.8 GHz (corresponding toRayleigh-scattered light), f1−f0−fB=1.5 GHz, f2−f0−fB=0.3 GHz (300 MHz)by the interference (multiplexing) in the optical coupler 22. Thus, ifthe band width B is selected to be 600 MHz or shorter, only theBrillouin-scattered light by the second pulse light can be outputtedfrom the light receiver 23. In this way, in this embodiment, an outputcorresponding to the Brillouin-scattered light by the second pulse lightis obtained by the light receiver 23 by causing the CW laser beam fromthe light source 11 to interact in the optical coupler 22 and suitablyselecting the band width B of the light receiver 23. If an absolutevalue of a variable x is expressed by abs[x], the band width B of thelight receiver 23 may be so set as to hold the following relationship(1).abs[f 1−f 0−fB]>>B>abs[f 2−f 0−fB]  (1).

The distributed lights are inputted to the light receiver 23 to bephotoelectrically converted. In order to cancel a noise created in thelight source 11, a balanced receiver (BR) is, for example, used as thelight receiver 23 in this embodiment. An output of the light receiver 23is multiplexed with a sinusoidal electrical signal having the frequencyfe and inputted from the oscillator 25 in the mixer 24, and inputted tothe BPF 26. For example, in this embodiment, the frequency fe is set at1.3 GHz, and the transmission characteristic of the BPF 26 is set suchthat a center frequency ωe is 1.0 GHz and a transmission frequency rangeis 1 MHz.

An output of the BPF 26 is amplified to a specified level and convertedfrom an analog signal into a digital signal in the A/D converter 28. Theresulting digital signal is squared for the conversion into a lightintensity Ps(t) corresponding to the Brillouin-scattered light.

Here, in the aforementioned case, a signal by the Brillouin-scatteredlight can be recorded even at abs[fe−ω]=1.0 GHz if a sampling rate ofthe A/D converter 28 is set at 5 GS.

It should be noted that “s” in Ps(t) denotes frequency and is Sb2 inFIG. 4. In this embodiment, $\begin{matrix}\begin{matrix}{s = {Sb2}} \\{= {\left( {{f2} - {fB}} \right) - {f0} + {fe}}}\end{matrix} & (2)\end{matrix}$Ps(t) is inputted to the buffer 29 to be temporarily saved there.

In this way, a spectrum Pfe corresponding to the frequency s shown byhatching in FIG. 5 can be obtained. FIG. 5 is a graph showing arelationship between the frequency of the oscillator and the spectra ofthe Brillouin-scattered light, wherein a horizontal axis representsfrequency and a vertical axis represent the intensity of theBrillouin-scattered light.

In this embodiment, dt=1 ns since being adjusted to RF amplitude of 1GHz. Thus, Ps(t) represents the light intensity of theBrillouin-scattered light created while the second pulse lightpropagates in the optical fiber 21 only for 1 ns. On the other hand,since the second pulse light propagates at a speed V of 200 nm/ns for 1ns, a propagation distance thereof in the optical fiber 21 is:dz=dt×V/2=10 cm. As a result, Ps(t) represents the light intensity ofthe Brillouin-scattered light every 10 cm in the optical fiber 21.

Accordingly, if a calculation is made as described later using Ps(t),the distortion and the temperature of the optical fiber 21 can bemeasured with a spatial resolution of 10 cm in this embodiment.

Next, a method for calculating the distortion and the temperature isdescribed. FIG. 6 is a diagram showing virtual small spaces and theoptical fiber.

In FIG. 6, Z-axis having an origin at the starting point of thereference light fiber portion 21-1 is set along the longitudinaldirection of the optical fiber 21. The reference light fiber portion21-1 is divided into m small spaces, and the detection light fiberportion 21-2 is divided into (n−1) small spaces. Further, the secondpulse light is thought to be virtually divided into m sections at veryshort time intervals dt. In this embodiment, the length of the smallspaces is 10 cm as described above. In this case, a Brillouin scatteringgain matrix is expressed as in equation (3) below. $\begin{matrix}{{{\begin{pmatrix}{a\left( {1,1} \right)} & {a\left( {1,2} \right)} & \ldots & {a\left( {1,m} \right)} & 0 & \ldots & \ldots & \ldots & 0 \\0 & {a\left( {2,2} \right)} & {a\left( {2,3} \right)} & \ldots & {a\left( {2,{m + 1}} \right)} & \ldots & \ldots & \ldots & 0 \\\vdots & ⋰ & \quad & \quad & \quad & \quad & \quad & \quad & \vdots \\0 & \ldots & {a\left( {i,i} \right)} & {a\left( {i,{i + 1}} \right)} & \ldots & {a\left( {i,{i + n - 1}} \right)} & 0 & \ldots & 0 \\\vdots & \quad & ⋰ & \quad & \quad & \quad & \quad & \quad & \vdots \\0 & \ldots & \quad & {a\left( {n,n} \right)} & {a\left( {n,{n + 1}} \right)} & \ldots & \ldots & \ldots & {a\left( {n,{m + n - 1}} \right)}\end{pmatrix}\quad}\begin{pmatrix}{{gs}(1)} \\{{gs}(2)} \\\vdots \\{{gs}(i)} \\\vdots \\{{gs}(m)} \\\vdots \\{{gs}\left( {m + n - 1} \right)}\end{pmatrix}} = \begin{pmatrix}{{Qs}(1)} \\{{Qs}(2)} \\\vdots \\{{Qs}(i)} \\\vdots \\{{Qs}(n)}\end{pmatrix}} & (3)\end{matrix}$

Here, Qs(i) (i: integer 1≦i<n) are variables determined by the lightintensities of the Brillouin-scattered light in a plurality of smallspaces z(i) to z(i+n−1) shown in FIG. 6 at the set frequency f2 of thesecond pulse light and by the light intensity of the second pulse lightincident on the optical fiber 21; gs(j) is a scattering gain coefficientof the j-th small space z(j) corresponding to the frequency s; and a(i,j) is a contribution ratio of the light intensity of theBrillouin-scattered light in the j-th small space z(j) (j=i to (i+m−1))to the variable Qs(i).

Qs(i) is expressed as in equation (4) in view of the light intensity ofthe Brillouin-scattered light by the second pulse light and apropagation loss of the second pulse light in the optical fiber 21:Qs(i)=lnPs(ti)+αL−η  (4)where α and L denote an attenuation coefficient of the optical fiber 21and the length of the optical fiber 21. η is a constant, but may be setat 0 for the simplicity.

When the frequency fe of the oscillator 25 is scanned at intervals of 5MHz, time domain spectra of the Brillouin-scattered light can bemeasured.

Further, if a(i, j) obtained in a certain section i using equations (3),(4) is expressed by a frequency axis, the scattering gain spectrum inthis section i can be obtained.

FIG. 7 is a graph showing scattering gain spectra, wherein a horizontalaxis represents frequency and a vertical axis represents scattering gaincoefficient. A solid-lined curve in FIG. 7 represents a measuredscattering gain spectrum A, whereas a broken-lined curve in FIG. 7 is ascattering gain spectrum B in the reference light fiber portion 21-1. Δgis a difference between a maximum value of the measured scattering gainspectrum A and a maximum value of the scattering gain spectrum B in thereference light fiber portion 21-1, and Δν is a difference between afrequency giving the maximum value of the measured scattering gainspectrum A and a frequency giving the maximum value of the scatteringgain spectrum B in the reference light fiber portion 21-1.

Equation (5) is known to hold if εi, Ti, εr, Tr denote the distortionand the temperature in the section i and the distortion and thetemperature of the reference light fiber portion 21-1, respectively.$\begin{matrix}{\left( \frac{\begin{matrix}{\Delta\quad\upsilon} \\{\Delta\quad g}\end{matrix}}{g_{r}} \right) = {\begin{pmatrix}{C\quad{ɛ\upsilon}} & {{CT}\quad\upsilon} \\{C\quad ɛ\quad P} & {CTP}\end{pmatrix}\begin{pmatrix}{\Delta ɛ} \\{\Delta\quad T}\end{pmatrix}}} & (5)\end{matrix}$

Here, Δε=εr−εi; ΔT=Tr−Ti; gr is a maximum gain coefficient of thereference light fiber portion 21-1; Cεν, CεP, CTν and CTP are constantspeculiar to the optical fibers 21.

In this way, the measurement results can be put into equations (3), (4)to obtain the scattering gain coefficient shift Δg and the frequencyshift Δν shown in FIG. 7, and the distortion εi=εr+Δε and thetemperature Ti=Tr+ΔT in the small spaces z(i) can be calculated usingequation (5).

Next, operations in the case of actually measuring the distortion andthe temperature using the distribution optical fiber sensor system 10are described.

FIG. 8 is a flowchart showing operations of the distribution opticalfiber sensor system according to the first embodiment.

First, the controlling/calculating unit 30 performs initial settings byinitializing the respective devices and causing the RF signal source 15and the oscillator 25 to operate (Step S11). Subsequently, thecontrolling/calculating unit 30 controls the controller 13 so that theaforementioned first and second pulse lights can be outputted from thelight frequency converter 14 (Step S12). The controlling/calculatingunit 30 then drives the light source 11 to cause it to emit a CW laserbeam. The emitted CW laser beam is caused to emerge out as the first andsecond pulse lights from the light frequency converter 14 by thecontroller 13 and the RF signal source 15 as described above (Step S13).The outputted first and second pulse lights act as described above inthe respective devices, and the Brillouin-scattered light by the secondpulse light is sampled (Step S14). Subsequently, the Brillouin-scatteredlight is sampled over a specified period to perform sampling over theentire length of the detection light fiber portion 21-2 (Step S15). If Ldenotes the entire length of the optical fiber 21, the specified periodis 2L/V since it is a time for the light to propagate back and forth inthe optical fiber 21.

Next, the controlling/calculating unit 30 repeats Steps S12 to S15 (StepS16) while shifting the frequency fe of the oscillator 25 by a specifiedamount within a frequency range where the Brillouin-scattered light ispredicted to be created (Step S16). In this way, data used to obtain themeasured scattering gain spectrum A shown in FIG. 7 is obtained in everysmall space of the optical fiber 21.

Here, in order to suppress the influence of the polarization, thecontrolling/calculating unit 30 may repeat Steps S12 to S16 whilerotating the polarizing surfaces of the first and second pulse lights bya specified angle by controlling the polarization controller 18, therebyobtaining average data used to obtain the measured scattering gainspectrum A shown in FIG. 7. Since this averages the polarization effectin the optical fiber 21 and enables a more precise characteristic curveC to be obtained, the distortion can be more precisely measured.

Next, the controlling/calculating unit 30 calculates the distortion andthe temperature in each small space with a high spatial resolution usingthe data saved in the buffer 29 in accordance with the aforementionedequations (3) to (5) (Step S17).

As described above, since the distribution optical fiber sensor system10 of this embodiment virtually divides the optical fiber 21 into thesmall spaces and calculates the distortion and the temperature based onthe Brillouin-scattered light in each small space, the distortion andthe temperature can be measured with a higher spatial resolution ascompared to conventional distribution optical fiber sensor systems. Thespatial resolution of this embodiment is determined by the speed of thelight in the optical fiber and the sampling rate. Since the distributionoptical fiber sensor system 10 of this embodiment causes the propagationof the second pulse light for creating the Brillouin-scattered light formeasurement after causing the propagation of the first pulse light, notransient phenomenon occurs in the Brillouin-scattered light formeasurement. Therefore, the distortion and the temperature can be moreprecisely measured.

Here, a technique of measuring the distortion and the temperature with ahigher resolution by improving Step S17 of the first embodiment isdescribed.

FIG. 9 is a flowchart showing operations in Step S17. FIGS. 10A and 10Bare graphs showing a frequency characteristic of a two-dimensionallow-pass filter. FIG. 10A shows the frequency characteristic of thetwo-dimensional low-pass filter during a certain period, wherein ahorizontal axis represents frequency and a vertical axis representsinput/output ratio. FIG. 10B shows the frequency characteristic of thetwo-dimensional low-pass filter at a certain frequency, wherein ahorizontal axis represents time and a vertical axis representsinput/output ratio. FIG. 11 is a graph showing a relationship betweenmeasurement points and interpolation points obtained from themeasurement points, FIG. 12 is a graph showing the matrix a(i, j) ofequation (3) in the case that a pulse light is rectangular, and FIG. 13is a graph showing a characteristic of a filter used in a CT processing.

In FIG. 9, the controlling/calculating unit 30 filters the measurementdata saved in the buffer 29 using the two-dimensional low-pass filterhaving a frequency characteristic Lp shown in FIG. 10 (Step S31). Here,a cutoff frequency νc shown in FIG. 10 depends on the line width of theBrillouin scattering and is 100 MHz, for example, if this line width is35 MHz. Subsequently, the controlling/calculating unit 30 interpolates adesired number data between two points where the measurement data areobtained, for example, by linear interpolation or nonlinearinterpolation (Step S32). FIG. 11 shows a state where data □ at twopoints between measurement data ◯ at two points are interpolated bylinear interpolation. Subsequently, the controlling/calculating unit 30applies CT filtering using a characteristic curve shown in FIG. 13 (StepS33). Then, the controlling/calculating unit 30 calculates thedistortion and the temperature in each small space in accordance withthe aforementioned equations (3) to (5), using the data after theoperations in Step s31 to S33 (Step S34). In this way, the distortionand the temperature can be measured with a higher resolution. In theaforementioned interpolation, the distortion and the temperature can bemeasured at a higher resolution of an interval between the data □ whichis three times shorter than an interval between the measurement data ◯determined by the A/D converter 28.

Next, another embodiment is described.

Construction of Second Embodiment

Although the distribution optical fiber sensor system 10 of the firstembodiment obtains the data used to obtain the measured scattering gainspectrum A shown in FIG. 7 in Step S16 shown in FIG. 8 by repeatingSteps S12 to S15 within a specified frequency range, a distributionoptical fiber sensor system 11 of a second embodiment obtains data usinga time-frequency analysis technique.

FIG. 14 is a diagram showing a construction of a distribution opticalfiber sensor system 110 according to the second embodiment. In FIG. 14,the distribution optical fiber sensor system 110 of the secondembodiment is provided with a light source 11, optical couplers 12, 19,22, a controller 13, a light frequency converter 14, an RF signal source15, a light amplifier 16, an optical isolator 17, a polarizationcontroller 18, an optical fiber 21, a light receiver 23, a BPF 41, anamplifier 42, an A/D converter 43, a time-frequency analyzer 44 and acontrolling/calculating unit 45.

Next, operations of the distribution optical fiber sensor system 110according to the second embodiment are described.

Operations of Second Embodiment

A CW laser beam having a frequency f0 and emitted from the light source11 is incident on the light frequency converter 14 via the opticalcoupler 12. The incident CW laser beam is converted into a first pulselight having a frequency f1=5 GHz and a pulse width of 100 ns, and asecond pulse light having a frequency f2=10.3 GHz and a pulse width of30 ns in the light frequency converter 14, which are then incident onthe light amplifier 16. There is a time interval of 2 ns between thefirst and second pulse lights. Thereafter, these first and second pulselights are incident on the optical fiber 21 via the light amplifier 16,the optical isolator 17, the polarization controller 18 and the opticalcoupler 19 while acting in the same manner as in the first embodiment.

A Brillouin-scattered light created in the optical fiber 21 by thesecond pulse light is inputted to a port “r” of the optical coupler 22via the optical coupler 19. The CW laser beam inputted to a port “q” andthe Brillouin-scattered light inputted to the port “r” are coupled inthe optical coupler 22 and distributed into two lights to be incident onthe light receiver 23.

Here, in the case of using an optical fiber of 10.7 GHz, the followingrelationships hold: f1−f0=5 GHz, f2−f0=10.7 GHz (corresponding toRayleigh-scattered light), f1−f0−fB=5.7 GHz, f2−f0−fB=0.4 GHz (400 MHz)by the interference (multiplexing) in the optical coupler 22. Thus, if aband width B is selected to be 2 GHz or shorter, only theBrillouin-scattered light by the second pulse light can be outputtedfrom the light receiver 23. The band width B of the light receiver 23may be so set as to hold equation (1) as in the first embodiment.

The distributed lights are inputted to the light receiver 23 to bephotoelectrically converted. An output of the light receiver 23 isinputted to the BPF 41. For example, in this embodiment, thetransmission characteristic of the BPF 41 is set such that a centerfrequency is 300 MHz and a transmission frequency range is 400 MHz.Thus, an output of the BPF 41 takes a maximum frequency of 500 MHz and aminimum frequency of 100 MHz.

The output of the BPF 41 is amplified to a specified level in theamplifier 42 and converted from an analog signal into a digital signalin the A/D converter 43. Here, in the aforementioned case, a samplingrate of the A/D converter 43 is set at 4×500=2000 MS or larger. In thisembodiment, this sampling rate is set at 8 GS. An output of the A/Dconverter 43 is inputted to the time-frequency analyzer 44.

FIG. 15 is a chart showing an exemplary waveform of an input to thetime-frequency analyzer 44, wherein a horizontal axis represents timeand a vertical axis represents amplitude. FIG. 16 is a graph showingfrequency spectra at time windows, wherein x-axis, y-axis and z-axis ofFIG. 16 represent time, frequency and intensity, respectively.

The time-frequency analyzer 44 sets a time window W1 of a specifiedperiod T and applies Fourier transform to data in this time window W1 toobtain a frequency spectrum Spw1. By successively shifting the timewindow W1 by a specified time dw to a time window W2, a time window W3,etc., frequency spectra Spw2, Spw3, . . . at the respective times areobtained in real time as shown in FIG. 16. Here, Spw2 is a frequencyspectrum at time (T+dw)/2 from Spw1.

For example, in this embodiment, the specified period T is set at 400ns, 3200 data are obtained during this period T, and Fourier transformis applied to the obtained data to obtain the frequency spectra.Further, the specified time dw is set at 10 ns, and the frequencyspectra at the respective times are obtained.

For example, a real-time spectrum analyzer or the like may be used assuch a time-frequency analyzer 44. Although the time-frequency analysistechnique is used to obtain the frequency spectra at the respectivetimes in this embodiment, wavelet transform may be used.

The controlling/calculating unit 45 measures a distortion created in theoptical fiber 21 and the temperature of the optical fiber 21 with aspatial resolution of 10 cm in accordance with equations (3) to (5) inthe same manner as in the first embodiment, using the frequency spectraat the respective times obtained in the time-frequency analyzer 44.

Next, operations in the case of actually measuring the distortion andthe temperature using the distribution optical fiber sensor system 110are described.

FIG. 17 is a flowchart showing the operations of the distributionoptical fiber sensor system 110 according to the second embodiment.

In FIG. 17, the controlling/calculating unit 45 first performs initialsettings by initializing the respective devices and causing the RFsignal source 15 to operate (Step S21). Subsequently, thecontrolling/calculating unit 45 controls the controller 13 so that theaforementioned first and second pulse lights can be outputted from thelight frequency converter 14 (Step S22). The controlling/calculatingunit 45 then drives the light source 11 to cause it to emit a CW laserbeam and causes the light frequency converter 14 to output the first andsecond pulse lights (Step S23). The outputted first and second pulselights act as described above in the respective devices, and theBrillouin-scattered light by the second pulse light is sampled (StepS24). Subsequently, the Brillouin-scattered light is sampled over aspecified period to perform sampling over the entire length of adetection light fiber portion 21-2 (Step S25).

In this way, frequency spectra as shown in FIG. 16, i.e., data used toobtain a scattering gain spectrum, are obtained in the respective smallspaces. Subsequently, the controlling/calculating unit 45 calculates thedistortion and the temperature for each small space with a high spatialresolution in accordance with the aforementioned equations (3) to (5),using the data obtained in the time-frequency analyzer 44 (Step S26).

In addition to the effects of the distribution optical fiber sensorsystem 10 of the first embodiment, the distribution optical fiber sensorsystem 110 of this embodiment has further effects of a shorter measuringtime and a real-time measurement since the time-frequency analysistechnique is used to obtain the data as described above, makingfrequency scanning unnecessary unlike the first embodiment. Therefore,even objects to be measured whose distortion and temperature dynamicallychange can be measured.

Next, still another embodiment is described.

Third Embodiment

In a distribution optical fiber sensor system 120 according to a thirdembodiment, a pump light comprised of a first and a second pulse lightsand a probe light are so incident on an optical fiber that the probelight faces in a propagating direction of the first and second pulselights, the probe light is amplified by Brillouin scattering caused bythe second pulse light, and the distortion and the temperature arecalculated by detecting the Brillouin-amplified probe light(Brillouin-scattered light).

FIG. 18 is a diagram showing a construction of a distribution opticalfiber sensor system 120 according to the third embodiment.

In FIG. 18, the distribution optical fiber sensor system 120 of thethird embodiment is provided with a light source 11, optical couplers12, 19, 22, 51, a controller 13, a light frequency converter 14, an RFsignal source 15, a light amplifier 16, an optical isolator 17, apolarization controller 18, an optical fiber 21, a light receiver 23, aLPF 52′, an amplifier 53, an A/D converter 54, a buffer 55 and acontrolling/calculating unit 56.

Next, operations of the distribution optical fiber sensor system 120according to the third embodiment are described.

Operations of Third Embodiment

A CW laser beam having a frequency f0 and emitted from the light source11 is incident on the light frequency converter 14 via the opticalcoupler 12. The incident CW laser beam is converted into a first pulselight having a frequency f1=12 GHz and a pulse width of 100 ns, and asecond pulse light having a frequency f2=10.8 GHz and a pulse width of30 ns in the light frequency converter 14, which are then incident onthe light amplifier 16. There is a time interval of 2 ns between thefirst and second pulse lights. Thereafter, these first and second pulselights are incident on one end of the optical fiber 21 via the lightamplifier 16, the optical isolator 17, the polarization controller 18and the optical coupler 19 while acting in the same manner as in thefirst embodiment.

On the other hand, the CW laser light distributed in the optical coupler12 and outputted from a port “d” of the optical coupler 12 is incidenton a port “j” of the optical coupler 51 and distributed into two lights.One of the distributed light is outputted from a port “1” and incidenton a port “q” of the optical coupler 22, whereas the other thereof isoutputted from a port “k” and incident on the other end of the opticalfiber 21. The CW laser beam incident on the other end of the opticalfiber 21 becomes the probe light.

The probe light (Brillouin-scattered light) amplified by Brillouinscattering of the second pulse light in the optical fiber 21 is incidenton a port “r” of the optical coupler 22 via the optical coupler 19. TheCW laser beam incident on the port “q” and the Brillouin-scattered lightincident on the port “r” are coupled in the optical coupler 22 anddistributed into two lights to be incident on the light receiver 23.Specifically, these distributed lights serve for a homodyne detection.

Here, in the case of using an optical fiber of 10.5 GHz, the followingrelationships hold: f1−f0=12 GHz, f2−f0=10.8 GHz, f1−f0−fB=1.5 GHz,f2−f0−fB=0.3 GHz (300 MHz) by the interference (multiplexing) in theoptical coupler 22. Thus, if a band width B is selected to be 1 GHz,only the Brillouin-scattered light by the second pulse light can beoutputted from the light receiver 23. The band width B of the lightreceiver 23 may be so set as to hold equation (1) as in the firstembodiment.

The distributed lights are inputted to the light receiver 23 to bephotoelectrically converted. An output of the light receiver 23 isinputted to the LPF 52′. For example, in this embodiment, thetransmission characteristic of the LPF 52′ is set such that atransmission frequency range is 100 MHz.

An output of the BPF 52′ is amplified to a specified level in theamplifier 52 and converted from an analog signal into a digital signalin the A/D converter 54. Here, in the aforementioned case, a samplingrate of the A/D converter 54 is set at 1600 MS or larger and convertedinto the intensity of the RF signal. In this embodiment, this samplingrate is set at 2 GS. The buffer 55 temporarily saves an output of theA/D converter 54. The controlling/calculating unit 30 controls therespective devices of the distribution optical fiber sensor system 120and measures a distortion created in the optical fiber 21 and thetemperature of the optical fiber 21 with a spatial resolution of 5 cm inaccordance with equations (3) to (5) in the same manner as in the firstembodiment using the data saved in the buffer 55.

Since operations in the case of actually measuring the distortion andthe temperature using the distribution optical fiber sensor system 120are the same as those of the distribution optical fiber sensor system 10of the first embodiment shown in FIG. 8, no description is giventhereon.

In addition to the effects of the distribution optical fiber sensorsystem 10 of the first embodiment, the distribution optical fiber sensorsystem 120 of this embodiment has further effects of a stronger signallight and a longer measuring distance since the probe light is used asdescribed above.

Next, further another embodiment is described.

Fourth Embodiment

Although the distribution optical fiber sensor system 120 of the thirdembodiment detects the Brillouin-scattered light by the homodynedetection, a distribution optical fiber sensor system 130 of a fourthembodiment detects the Brillouin-scattered light by a heterodynedetection.

FIG. 19 is a diagram showing a construction of the distribution opticalfiber sensor system 130 according to the fourth embodiment. In FIG. 19,the distribution optical fiber sensor system 130 of the fourthembodiment is provided with a light source 11, optical couplers 12, 19,22, 51, a controller 13, light frequency converters 14, 57, an RF signalsource 15, a light amplifier 16, an optical isolator 17, a polarizationcontroller 18, an optical fiber 21, a light receiver 23, a BPF 52, anamplifier 53, an A/D converter 54, a buffer 55 and acontrolling/calculating unit 56.

Next, operations of the distribution optical fiber sensor system 130according to the fourth embodiment are described.

Operations of Fourth Embodiment

A CW laser beam having a frequency f0 and emitted from the light source11 is incident on the light frequency converter 14 via the opticalcoupler 12. The incident CW laser beam is converted into a first pulselight having a frequency f1=12 GHz and a pulse width of 100 ns, and asecond pulse light having a frequency f2=10.8 GHz and a pulse width of30 ns in the light frequency converter 14, which are then incident onthe light amplifier 16. There is a time interval of 2 ns between thefirst and second pulse lights. Thereafter, these first and second pulselights are incident on one end of the optical fiber 21 via the lightamplifier 16, the optical isolator 17, the polarization controller 18and the optical coupler 19 while acting in the same manner as in thefirst embodiment.

On the other hand, the CW laser light distributed in the optical coupler12 and outputted from a port “d” of the optical coupler 12 is incidenton a port “j” of the optical coupler 51 and distributed into two lights.One of the distributed light is outputted from a port “1”, has thefrequency thereof converted into a specified frequency (fao) in thelight frequency converter 57, and is incident on a port “q” of theoptical coupler 22, whereas the other thereof is outputted from a port“k” and incident on the other end of the optical fiber 21. The CW laserbeam incident on the other end of the optical fiber 21 becomes a probelight.

The probe light (Brillouin-scattered light) amplified by Brillouinscattering of the second pulse light in the optical fiber 21 is incidenton a port “r” of the optical coupler 22 via the optical coupler 19. TheCW laser beam of a specified frequency incident on the port “q” and theBrillouin-scattered light incident on the port “r” are coupled in theoptical coupler 22 and distributed into two lights to be incident on thelight receiver 23. Since the specified frequency (fao) is set at 120MHz, this light serves for a heterodyne detection.

The distributed lights are inputted to the light receiver 23 to bephotoelectrically converted. An output of the light receiver 23 isinputted to the BPF 52. For example, in this embodiment, thetransmission characteristic of the BPF 52 is set such that a centerfrequency is 120 MHz and a transmission frequency range is set at 1 MHz.

An output of the BPF 52 is amplified to a specified level in theamplifier 52 and converted from an analog signal into a digital signalin the A/D converter 54. Here, in the aforementioned case, a samplingrate of the A/D converter 54 is set at 500 MS or larger. In thisembodiment, this sampling rate is set at 1 GS. An output of the A/Dconverter 54 is temporarily saved in the buffer 55. Thecontrolling/calculating unit 30 controls the respective devices of thedistribution optical fiber sensor system 130 and measures a distortioncreated in the optical fiber 21 and the temperature of the optical fiber21 with a spatial resolution of 10 cm in accordance with equations (3)to (5) in the same manner as in the first embodiment using the datasaved in the buffer 55.

Since operations in the case of actually measuring the distortion andthe temperature using the distribution optical fiber sensor system 130are the same as those of the distribution optical fiber sensor system 10of the first embodiment shown in FIG. 8, no description is giventhereon.

In addition to the effects of the distribution optical fiber sensorsystem 10 of the first embodiment, the distribution optical fiber sensorsystem 130 of this embodiment has further effects of an elongatedmeasurement range and a higher precision since the probe light is usedas described above.

Next, further another embodiment is described.

Fifth Embodiment

A distribution optical fiber sensor system 140 according to a fifthembodiment is so constructed as to amplify a Brillouin-scattered lightby a second pulse light by looping it and to measure a distortion and atemperature by detecting the amplified Brillouin-scattered light.

FIGS. 20A and 20B are diagrams showing the distribution optical fibersensor system 140 of the fifth embodiment, wherein FIG. 20A shows aconstruction thereof and FIG. 20B is a timing chart showing operationsof a optical gate switch and a light frequency converter. FIG. 21 is adiagram showing a physical process of the fifth embodiment.

In FIG. 20A, the distribution optical fiber sensor system 140 of thefifth embodiment is provided with a light source 11, optical couplers12, 19, 22, 61, 64, a controller 13, light frequency converters 14, 57,an RF signal source 15, light amplifiers 16, 63, an optical isolator 17,a polarization controller 18, an optical fiber 21, a light receiver 23,an optical gate switch 62, a BPF 65, an amplifier 66, an A/D converter67, a buffer 68 and a controlling/calculating unit 69. The optical gateswitch 62 is an optical switch for transmitting (ON) or shutting off(OFF) an incident light in accordance with a control signal.

Next, operations of the distribution optical fiber sensor system 140according to the fifth embodiment are described.

Operations of Fifth Embodiment

A CW laser beam having a frequency f0 and emitted from the light source11 is incident on the light frequency converter 14 via the opticalcoupler 12. The incident CW laser beam is converted into a first pulselight having a frequency f1=12 GHz and a pulse width of 100 ns, and asecond pulse light having a frequency f2=10.8 GHz and a pulse width of30 ns in the light frequency converter 14, which are then incident onthe light amplifier 16 at timings shown in FIG. 20B. There is a timeinterval of 2 ns between the first and second pulse lights. Thereafter,these first and second pulse lights are incident on one end of theoptical fiber 21 via the light amplifier 16, the optical isolator 17,the polarization controller 18 and the optical coupler 19 while actingin the same manner as in the first embodiment.

On the other hand, the CW laser light distributed in the optical coupler12 and outputted from a port “d” of the optical coupler 12 is incidenton a port “n” of the optical coupler 61 and distributed into two lights.One of the distributed lights is outputted from a port “o” and isincident on a port “v” of the optical coupler 64 via the optical gateswitch 62 at a timing shown in FIG. 20B, whereas the other thereof isoutputted from a port “p”, has the frequency thereof converted into aspecified frequency in the light frequency converter 57 and is incidenton a port “q” of the optical coupler 22.

Further, the CW laser beam incident on the port “v” of the opticalcoupler 64 is outputted from a port “w” and incident on the other end ofthe optical fiber 21. The CW laser beam incident on the other end of theoptical fiber 21 becomes a probe light. On the other hand, the probelight (Brillouin-scattered light) amplified by Brillouin scattering ofthe second pulse light in the optical fiber 21 is outputted from a port“f” of the optical coupler 19, amplified to a specified level in thelight amplifier 63, and incident on a port “u” of the optical coupler64. The Brillouin-scattered light incident on the port “u” isdistributed in the optical coupler 64, and the Brillouin-scattered lightoutputted from the port “w” is incident on the other end of the opticalfiber 21. The CW laser beam (probe light) outputted from the opticalgate switch 62 in this way circulates a loop of the optical coupler64→the optical fiber 21→the optical coupler 19→the light amplifier63→the optical coupler 64 by a specified number of times. The first andsecond pulse lights are outputted from the light frequency converter 14during this circulation at every interval of a circulation time TLoopshown in FIG. 20B under the control of the controlling/calculating unit69 so as to interact with the circulating probe light at the sametiming. Because of the repeated interaction in the optical fiber 21 bycirculating the loop by the specified number of times, the distortionand the temperature of the optical fiber 21 can be better reflected andthe polarized states of the second pulse light and the probe light canbe averaged. Therefore, the dynamically changing distortion andtemperature can also be dealt with.

On the other hand, the other Brillouin-scattered light distributed inthe optical coupler 64 is incident on a port “r” of the optical coupler22. The CW laser beam incident on the port “q” and theBrillouin-scattered light incident on the port “r” are coupled in theoptical coupler 22 and distributed into two lights. These distributedlights are respectively inputted to the light receiver 23 to bephotoelectrically converted. Here, an output of the light receiver 23 isoutputted to the BPF 65 at a timing after the Brillouin-scattered lightcirculates the specified time of times, e.g., 40 times under the controlof the controlling/calculating unit 69.

Here, in the case of using an optical fiber of 10.5 GHz, the followingrelationships hold: f1−f0=12 GHz, f2−f0=10.8 GHz, f1−f0−fB=1.5 GHz,f2−f0−fB=0.3 GHz (300 MHz) by the interference (multiplexing) in theoptical coupler 22. Thus, if a center frequency and a transmissionfrequency range are selected to be 120 MHz and 1 MHz, respectively, onlythe Brillouin-scattered light by the second pulse light can be outputtedfrom the light receiver 23.

An output of the BPF 65 is amplified to a specified level in theamplifier 66 and converted from an analog signal into a digital signalin the A/D converter 67. Here, in the aforementioned case, a samplingrate of the A/D converter 67 is set at 400 MS or larger. According tothe procedure described with reference to FIG. 9, interpolation issubstantially applied at 2000 MS. In this embodiment, this sampling rateis set at 2 GS. The buffer 55 temporarily saves an output of the A/Dconverter 54. The controlling/calculating unit 30 control the respectivedevices of the distribution optical fiber sensor system 10 and measuresa distortion created in the optical fiber 21 and the temperature of theoptical fiber 21 with a spatial resolution of 5 cm in accordance withequations (3) to (5) as in the first embodiment using the data saved inthe buffer 55.

Here, Qs(i) in equation (3) is obtained as follows. In FIG. 21, Z-axishaving an origin at the starting point of a reference light fiberportion 21-1 is set along the longitudinal direction of the opticalfiber 21. Further, D_(L) in FIG. 21 collectively represents a loss inthe loop of the optical coupler 64→the optical fiber 21→the opticalcoupler 19→the light amplifier 63→the optical coupler 64. The probelight incident on Z=L and circulating in the k-th time is expressed asin equation (6). $\begin{matrix}{{\ln\quad{P_{P}(0)}^{(k)}} = {{- {\int_{zo}^{ze}{{R^{(k)}(Z)}{\gamma\left( {Z,S} \right)}{S_{S}(Z)}\quad{\mathbb{d}Z}}}} - {\alpha_{P}L} + {\ln\quad{P_{P}(L)}^{({k - l})}}}} & (6)\end{matrix}$where Ps^((k)) (z) represents the intensity of the probe light having afrequency s at position z in the k-th time. R^((k)) (z) represents apolarization coefficient of the second pulse light and the probe lightat the position z in the k-th time and is a random number from 0 to 1 ina single-mode optical fiber. γ(z, s) represents a Brillouinamplification coefficient corresponding to the frequency s at theposition z and basically depends on the distortion and the temperatureonce the wavelength is determined. αs represents an attenuationcoefficient of the optical fiber 21, and Sp(z) is an energy density ofthe second pulse light and expressed as in equation (7). $\begin{matrix}{{{Sp}(z)} = \frac{{Ps}(z)}{Aeff}} & (7)\end{matrix}$where Aeff is an effective cross sectional area of the optical fiber 21.Further, equation (8) holds from FIG. 21.Ps ^((k-1))(L)=D _(L) G _(E) ^((k-1))(P _(S) ^((k-2))(0)   (8)

If equations (6) to (8) are put together and the detection light fiberportion 21-2 is equally divided into a plurality of small sectionshaving a length dz and put into a differential equation, followingequations (9) to (12) hold. $\begin{matrix}{{\sum\limits_{j = {je}}^{j0}{a_{ij}R_{i}^{k}r_{j,s}}} = Q_{i,s}^{k}} & (9) \\{a_{i,j} = {\frac{P_{s}^{j}{dz}}{Aeff}{\exp\left( {{- \alpha_{P}}Z_{j}} \right)}}} & (10) \\{R_{i}^{k} = \frac{\sum\limits_{{nk} = 1}^{k}R_{j}^{({nk})}}{k - 1}} & (11) \\{Q_{i,s}^{K} = {{\frac{1}{k - 1}{\ln\left( \frac{P_{s}^{0}}{P_{s}^{(k)}(L)} \right)}} - {\alpha\quad{sL}\quad\frac{k - 2}{k - 1}{\ln(\beta)}}}} & (12)\end{matrix}$where suffixes i, j are the numbers of the small sections. As can beseen from equation (11), R_(j) ^(k) is an average value of randomnumbers. Thus, the values of the respective small sections canapproximate to the same constant if the number of circulation k is madelarger.

In addition to the effects of the distribution optical fiber sensorsystem 10 of the first embodiment, the distribution optical fiber sensorsystem 140 of this embodiment has further effects that the distortionand temperature of the optical fiber 21 can be better reflected and thepolarized states of the second pulse light and the probe light can beaveraged since the Brillouin-scattered light is looped as describedabove. Therefore, the dynamically changing distortion and temperaturecan also be dealt with.

On the other hand, in the first to fifth embodiments, a lateral pressureacts on the optical fiber 21 at the same time it acts on the structure1, thereby causing a distortion in the optical fiber in radialdirection. This radial distortion can be obtained by calculating thelateral pressure in each small section by comparing an averagescattering gain spectrum in relation to various polarized lights(average scattering gain spectrum) and a scattering gain spectrum inrelation to a certain specific polarized light (specific scattering gainspectrum).

FIG. 22 is a flowchart in the case of calculating the lateral pressure.In FIG. 22, a polarizing surface of the polarization controller 18 isfirst adjusted to a suitable polarizing surface (Step S41), and ascattering gain spectrum is obtained in Steps S12 to S16 described withreference to FIG. 8 (Step S42). Steps S41 and S42 are repeated by aspecified number of times suitable to obtain an average while changingthe polarizing surface (Step S43). After these Steps are repeated by aspecified number of times, an average scattering gain spectrum isobtained by averaging the scattering gain spectra corresponding to therespective polarizing surfaces (Step S44).

Subsequently, the polarizing surface of the polarization controller 18is maintained at a specific polarizing surface (Step S45), and ascattering gain spectrum corresponding to this specific polarizingsurface (specific scattering gain spectrum) is calculated by Steps S12to S16 (Step S46). A polarization coefficient is obtained from theaverage scattering gain spectrum and the specific scattering gainspectrum to calculate the lateral pressure (Step S47).

In this way, the radial distortion of the optical fiber 21 can also bemeasured by using the distribution optical fiber sensor systems of thefirst to fifth embodiments.

As described above, a distribution optical fiber sensor systemcomprises: an optical fiber for sensing to be placed on an object to bemeasured, a light source for emitting a first pulse light having a pulsewidth longer than a transient response of an acoustic phonon andemitting a second pulse light in succession to the first pulse after atime interval during which the vibration of the acoustic phonon issubstantially maintained to supply the first and second pulse lights tothe optical fiber, a detector for detecting a scattering gain spectrumof a Brillouin-scattered light created in the optical fiber by thesecond pulse light at time intervals corresponding to twice the timeobtained by equally dividing the pulse width of the second pulse light,and a calculator for calculating a distortion and/or a temperature basedon the respective scattering gain spectra at the respective timeintervals for small sections of the optical fiber corresponding to therespective scattering gain spectra at the respective time intervals.

In the above distribution optical fiber sensor system, the detector maybe provided with an optical coupler for multiplexing a light of aspecified frequency and the Brillouin-scattered light from the opticalfiber; a light receiver for receiving and photoelectrically converting alight outputted from the optical coupler; an oscillator for oscillatingan electrical signal of a specified frequency; a mixer for multiplexingan output of the light receiver and an output of the oscillator; aband-pass filter for passing an output of the mixer within a specifiedfrequency band; a buffer for saving an output of the band-pass filter;and a controller for sweeping a specified frequency of the oscillatorwithin such a range where the scattering gain spectra can be obtained.Further, in such a distribution optical fiber sensor system, thedetector may be further provided with an interpolating device forinterpolating data between two outputs using the two outputs of theband-pass filter saved in the buffer.

Further, in the above distribution optical fiber sensor system, thedetector includes an optical coupler for multiplexing a light of aspecified frequency and the Brillouin-scattered light from the opticalfiber; a light receiver for receiving and photoelectrically converting alight outputted from the optical coupler; a band-pass filter for passingan output of the light receiver within a specified frequency band; and atime-frequency analyzer for applying a time-frequency analysis to anoutput of the band-pass filter.

Further, in the above distribution optical fiber sensor system, thedetector may be provided with an incidence device for causing a light ofa specified frequency to be so incident on the optical fiber as to facethe second pulse light; an optical coupler for multiplexing the light ofthe specified frequency and the Brillouin-scattered light from theoptical fiber; a light receiver for receiving and photoelectricallyconverting a light outputted from the optical coupler; a band-passfilter for passing an output of the light receiver within a specifiedfrequency band; and a controller for sweeping the specified frequency ofthe light within such a range where the scattering gain spectra can beobtained. In this distribution optical fiber sensor system, the detectormay be further provided with a light frequency converter for convertingthe frequency of the light of the specified frequency to conduct aheterodyne detection. Further, in these distribution optical fibersensor systems, the light of the specified frequency and the secondpulse light interact with each other a plurality of times in the opticalfiber. Further, in such a distribution optical fiber sensor system, thecalculator further calculates a lateral pressure acting on the object tobe measured based on the respective scattering gain spectra at therespective time intervals for the respective small sections of theoptical fiber corresponding to the respective scattering gain spectra atthe respective time intervals.

Further, in the above distribution optical fiber sensor system, thedetector may be provided with a first optical coupler for distributing alight of a specified frequency into two lights; an optical switch forpassing or shutting off one of the lights distributed in the firstoptical coupler; a second optical coupler for multiplexing a light fromthe optical switch and a Brillouin-scattered light from the opticalfiber, distributing the multiplexed light into two lights, and causingone of the distributed lights to be incident on the optical fiber; athird optical coupler for multiplexing the other of the lightsdistributed in the first optical coupler and the other of the lightsdistributed in the second light coupler; a light receiver for receivingand photoelectrically converting a light outputted from the thirdoptical coupler; a band-pass filter for transmitting an output of thelight receiver within a specified frequency band; and a time-frequencyanalyzer for applying a time-frequency analysis to an output of theband-pass filter.

Since the above distribution optical fiber sensor systems cause thepropagation of the second pulse light for creating theBrillouin-scattered light for measurement after causing the propagationof the first pulse light, no transient phenomenon occurs in theBrillouin-scattered light for measurement. Therefore, the distortion andthe temperature can be more precisely measured than before.

As this invention may be embodied in several forms without departingfrom the spirit of essential characteristics thereof, the presentembodiment is therefore illustrative and not restrictive, since thescope of the invention is defined by the appended claims rather than bythe description preceding them, and all changes that fall within metesand bounds of the claims, or equivalence of such metes and bounds aretherefore intended to embraced by the claims.

1. A distribution optical fiber sensor system, comprising: an opticalfiber for sensing to be placed on an object to be measured, a lightsource for emitting a first pulse light having a pulse width longer thana transient response of an acoustic phonon and emitting a second pulselight in succession to the first pulse after a time interval duringwhich the vibration of the acoustic phonon is substantially maintainedto supply the first and second pulse lights to the optical fiber, adetector for detecting a scattering gain spectrum of aBrillouin-scattered light created in the optical fiber by the secondpulse light at time intervals corresponding to twice the time obtainedby equally dividing the pulse width of the second pulse light, and acalculator for calculating a distortion and/or a temperature based onthe respective scattering gain spectra at the respective time intervalsfor small sections of the optical fiber corresponding to the respectivescattering gain spectra at the respective time intervals.
 2. Adistribution optical fiber sensor system according to claim 1, whereinthe detector includes: an optical coupler for multiplexing a light of aspecified frequency and the Brillouin-scattered light from the opticalfiber, a light receiver for receiving and photoelectrically converting alight outputted from the optical coupler, an oscillator for oscillatingan electrical signal of a specified frequency, a mixer for multiplexingan output of the light receiver and an output of the oscillator, aband-pass filter for passing an output of the mixer within a specifiedfrequency band, a buffer for saving an output of the band-pass filter,and a controller for sweeping a specified frequency of the oscillatorwithin such a range where the scattering gain spectra can be obtained.3. A distribution optical fiber sensor system according to claim 2,wherein the detector further includes an interpolating device forinterpolating data between two outputs using the two outputs of theband-pass filter saved in the buffer.
 4. A distribution optical fibersensor system according to claim 1, wherein the detector includes: anoptical coupler for multiplexing a light of a specified frequency andthe Brillouin-scattered light from the optical fiber, a light receiverfor receiving and photoelectrically converting a light outputted fromthe optical coupler, a band-pass filter for passing an output of thelight receiver within a specified frequency band, and a time-frequencyanalyzer for applying a time-frequency analysis to an output of theband-pass filter.
 5. A distribution optical fiber sensor systemaccording to claim 1, wherein the detector includes: an incidence devicefor causing a light of a specified frequency to be so incident on theoptical fiber as to face the second pulse light, an optical coupler formultiplexing the light of the specified frequency and theBrillouin-scattered light from the optical fiber, a light receiver forreceiving and photoelectrically converting a light outputted from theoptical coupler, a band-pass filter for passing an output of the lightreceiver within a specified frequency band, and a controller forsweeping the specified frequency of the light within such a range wherethe scattering gain spectra can be obtained.
 6. A distribution opticalfiber sensor system according to claim 5, wherein the detector furtherincludes a light frequency converter for converting the frequency of thelight of the specified frequency to conduct a heterodyne detection.
 7. Adistribution optical fiber sensor system according to claim 5, whereinthe light of the specified frequency and the second pulse light interactwith each other a plurality of times in the optical fiber.
 8. Adistribution optical fiber sensor system according to claim 7, whereinthe calculator further calculates a lateral pressure acting on theobject to be measured based on the respective scattering gain spectra atthe respective time intervals for the respective small sections of theoptical fiber corresponding to the respective scattering gain spectra atthe respective time intervals.
 9. A distribution optical fiber sensorsystem according to claim 1, wherein the detector includes: a firstoptical coupler for distributing a light of a specified frequency intotwo lights, an optical switch for passing or shutting off one of thelights distributed in the first optical coupler, a second opticalcoupler for multiplexing a light from the optical switch and aBrillouin-scattered light from the optical fiber, distributing themultiplexed light into two lights, and causing one of the distributedlights to be incident on the optical fiber, a third optical coupler formultiplexing the other of the lights distributed in the first opticalcoupler and the other of the lights distributed in the second lightcoupler, a light receiver for receiving and photoelectrically convertinga light outputted from the third optical coupler, a band-pass filter fortransmitting an output of the light receiver within a specifiedfrequency band, and a time-frequency analyzer for applying atime-frequency analysis to an output of the band-pass filter.